Utilitarianism is the idea that the moral worth of an action is solely determined by its contribution to overall utility in maximizing happiness or pleasure as summed among all people. It is, then, the total utility of individuals which is important here, the greatest happiness for the greatest number of people. Utility, after which the doctrine is named, is a measure in economics of the relative satisfaction from, or desirability of, the consumption of goods. Utilitarianism can thus be described as a quantitative and reductionistic approach to Ethics.

Utilitarianism starts from the basis that pleasure and happiness are intrinsically valuable, that pain and suffering are intrinsically disvaluable, and that anything else has value only in its causing happiness or preventing suffering (i.e. "instrumental", or as means to an end). This focus on happiness or pleasure as the ultimate end of moral decisions, makes it a type of Hedonism (and it is sometimes known as Hedonistic Utilitarianism).

Utilitarians support equality by the equal consideration of interests, and they reject any arbitrary distinctions as to who is worthy of concern and who is not, and any discrimination between individuals. However, it does accept the idea of declining marginal utility, which recognizes that the same thing furthers the interests of a well-off individual to a lesser degree than it would the interests of a less well-off individual.

It is a form of Consequentialism (in that the moral worth of an action is determined by its outcome or consequence - the ends justify the means), as opposed to Deontology (which disregards the consequences of performing an act, when determining its moral worth), and to Virtue Ethics (which focuses on character, rather than rules or consequences).

The origins of Utilitarianism are often traced back to the Epicureanism of the followers of the Greek philosopher Epicurus. It can be argued that David Hume and Edmund Burke were proto-Utilitarians.

But as a specific school of thought, it is generally credited to the English philosopher Jeremy Bentham. Bentham found pain and pleasure to be the only intrinsic values in the world, and this he derived the rule of utility: that the good is whatever brings the greatest happiness to the greatest number of people. Bentham himself, however, attributed the origins of the theory to Joseph Priestley (1733 - 1804), the English scientist, theologian and founder of Unitarianism in England.

Bentham's foremost proponent was James Mill (1773 - 1836) and his son John Stuart Mill, who was educated from a young age according to Bentham's principles. In his famous 1861 short work, "Utilitarianism", John Stuart Mill both named the movement and refined Bentham's original principles. Mill argued that cultural, intellectual and spiritual pleasures are of greater value than mere physical pleasure as valued by a competent judge (which, according to Mill, is anyone who has experienced both the lower pleasures and the higher).

In his essay "On Liberty" and other works, Mill argued that Utilitarianism requires that any political arrangements satisfy the liberty principle (or harm principle), according to which the only purpose for which power can be rightfully exercised over any member of a civilized community against his will, is to prevent harm to others, a cornerstone of the principles of Liberalism and Libertarianism. Some Marxist philosophers have also used these principles as arguments for Socialism.

The classic Utilitarianism of Bentham and Mill influenced many other moral philosophers and the development of many different types of Consequentialism.

It has been argued that measuring and comparing happiness among different people is impossible, not only in practice, but even in principle. Defenders argue that the same problem is sucessfully overcome in everyday life, and that rough estimates are usually sufficient.

Another dilemma of Utilitarianism is that the pleasure of a sadist should have the same importance as the pleasure of an altruist, although proponents have countered that sadists are relatively few and so their effective influence would be minimal, and that the hurt suffered by others would counterbalance any pleasure registered by the sadist. Furthermore, the sadist's pleasure is superficial and temporary, thus it is detrimental to the sadist's long term well-being.

Another argument is that sometimes a long time is needed to weigh all the evidence and reach a definite conclusion on the relative costs and benefits of an action. Utilitarians admit that certain knowledge of consequences is sometimes impossible, but argue that best estimates of the consequences or predictions based on the past are usually sufficient.

A very specific argument against Utilitarianism has been put forward on the grounds that Determinism is either true or false: if it is true, then we have no real choice over our actions; if it is false, then the consequences of our actions are unpredictable, not least because they will depend on the actions of others whom we cannot predict.

Utilitarianism has been criticized for only looking at the results of actions, not at the desires or intentions which motivate them, which many people also consider important. Thus, an action intended to cause harm but that inadvertently causes good results would be judged equal to the result from an action done with good intentions.

Utilitarians may argue that justification of slavery, torture or mass murder would require unrealistically large benefits to outweigh the direct and extreme suffering to the victims, as well as taking into consideration of the indirect impact of social acceptance of inhumane policies (e.g. general anxiety and fear might increase for all if human rights are commonly ignored).

Other critics have made objections to the following: the right and wrong dichotomy implicit in Utilitarianism, whereby a "good" act (e.g. a charitable donation) may be branded as a wrong action (e.g. if there is an alternative donation to a more efficient charity); Utilitarianism does not take account of the fact that human nature is dynamic and changing, so the concept of a single utility for all humans is one-dimensional and not useful; Utilitarians have no ultimate justification for primarily valuing pleasure, other than the tautological on that "this is the way it should be".

Some Consequentialists consider that, although happiness an important consequence, other consequences such as justice or equality should also be valued and taken into consideration, regardless of whether they increase happiness or not.

• Act Utilitarianism (or Case Utilitarianism) states that, when faced with a choice, we must first consider the likely consequences of potential actions in that particular case and, from that, choose to do what we believe will generate the most overall happiness. Act Utilitarians may follow certain rules of thumb (heuristics) to save time or cost although, if the consequences can be calculated relatively clearly, exactly and easily, then such rules of thumb can be ignored, and the choice treated on a case by case basis.

• Rule Utilitarianism states that, when faced with a choice, we must look at potential rules of action to determine whether the generalized rule produces more happiness than otherwise, if it were to be constantly followed. Thus, an action should only be carried out if it follows a rule that morally should be followed at all times. Rule Utilitarians may agree that there are some general exception rules that allow the breaking of other rules if this increases happiness (e.g. the exception of self-defence to overcome the general rule never to kill a human), although critics argue that this logically just reduces to Act Utilitarianism.

• Two-Level Utilitarianism states that normally we should use "intuitive" moral thinking, in the form of Rule Utilitarianism, because it usually maximizes happiness. However, there are some times when we must ascend to a higher "critical" level of reflection in order to decide what to do, and must think as an Act Utilitarian would. This method is based on the view that, although Act Utilitarianism may be preferable in theory, usually it is too difficult to perfectly predict consequences, and so we require moral guidelines or rules in day to day life.

• Motive Utilitarianism states that our initial moral task is to inculcate motives within ourselves (by means of teaching and repetition) that will be generally useful across the spectrum of the actual situations we are likely to encounter, rather than hypothetical examples which are unlikely to occur. It can be thought of as a hybrid between Act and Rule Utilitarianism, but it also attempts to take into account how human beings actually function psychologically.

• Total Utilitarianism advocates measuring the utility of a population based on the total utility of its members. However, it has been argued that this leads to a "repugnant conclusion", in which an enormous population whose individual lives are barely worth living is considered preferable to a smaller population with good lives.

• Average Utilitarianism advocates measuring the utility of a population based on the average utility of that population. The drawback here is known as the "mere addition paradox", where bringing a moderately happy person in a very happy world would be seen as an immoral act, or the logical implication that it would be a moral good to eliminate all people whose happiness is below average, as this would raise the average happiness.

• Negative Utilitarianism requires us to promote the least amount of evil or harm, or to prevent the greatest amount of suffering, for the greatest number (as opposed to the general, or positive, Utilitiarian rule of the greatest amount of good for the greatest number). The justification for Negative Utilitarianism is that the greatest harms are more consequential than the greatest goods, and so should have more influence on moral decision-making. Critics have argued that the ultimate aim of Negative Utilitarianism would therefore logically be to engender the quickest and least painful method of killing the entirety of humanity, as this would effectively minimize suffering, although more moderate proponents would obviously not propose that.

• Sentient Utilitarianism states that the well-being of all sentient beings (i.e. conscious beings who feel pain, including therefore some non-human animals) deserve equal consideration with that given to human beings, when making moral decisions in a Utilitarian context.